Recently, optical recording media such as the CD, DVD and the like have been widely used as recording media for recording a large volume of digital data.
These optical recording media can be roughly classified into so-called ROM type optical recording media such as the CD-ROM and the DVD-ROM that do not enable writing and rewriting of data, so-called write-once type optical recording media such as the CD-R and DVD-R that enable writing but not rewriting of data, and so-called data rewritable optical recording media such as the CD-RW and DVD-RW that enable rewriting of data.
Among these, write-once type optical recording disks such as the CD-R and DVD-R using an organic dye as a material for a recording layer have been widely used.
Since the CD-R in which data can be recorded and from which data can be reproduced using a laser beam having a near-infrared wavelength can prevent unauthorized alteration of recorded information and is low in cost, it is widely accepted in the market.
On the other hand, demand for increased recording density enabling motion picture images to be recorded over a long period of time has led to the development and wide use of a DVD-R in which data can be recorded and from which data can be reproduced using a laser beam of infrared wavelength. Specifically, in the DVD-R, recording capacity (4.7 GB/surface) which is six to eight times that of the CD has been achieved by shortening the wavelength of the laser beam used to record and reproduce data from 780 nm for the CD to 650 nm and increasing the numerical aperture NA of the optical system from 0.45 for the CD to 0.6. The DVD-R can record about two hours of ordinary television signals.
To enable recording of data in optical recording disks at still higher density, the wavelength of the laser beam used for recording and reproducing data has recently been made much shorter and the numerical aperture of the optical system made much larger. This has led to the development of a next-generation type optical recording disk suitable for a system using bluish-violet laser beam having a wavelength of 390 to 420 nm and an objective lens system whose numerical aperture NA is equal to or larger than 0.76. This use of a short wavelength laser beam is expected to markedly increase the recording density of optical recording disks.
Various organic dye materials have been proposed for use in the recording layer of the CD-R and the DVD-R but the only ones that have been put to practical use are those which have an absorption spectrum on the long wavelength side that coincides with the wavelength region of the laser beam used for recording and reproducing data and which have a refractive index n of higher than 2.0 and a suitable extinction coefficient k of higher than 0.01 and lower than 0.10 on the long wavelength side of the absorption spectrum thereof.
When the laser beam for recording and reproducing data is projected onto a recording layer, the organic dye material contained in the recording layer absorbs the laser beam to be melted or decomposed. As a result, the refractive index is greatly changed from a high refractive index to a low refractive index to form a record pit, thereby recording data in the recording layer. When data recorded in the recording layer are to be reproduced, the laser beam for recording and reproducing data is projected onto the recording layer and data are read utilizing the difference in reflective coefficients with respect to the laser beam between the record pit and unrecorded regions around the record pit.
In the CD-R or the DVD-R, a high reflective coefficient is required for compatibility with the CD-ROM or DVD-ROM, because these media have a high reflective coefficient. However, since a high reflective coefficient cannot be achieved using only an organic dye material having a refractive index n higher than 2.0 and an extinction coefficient k higher than 0.01 and lower than 0.10, a metal reflective layer having a high reflective coefficient is provided on the opposite side from the laser beam incidence side with respect to the recording layer. Since the CD-R and DVD-R thus require a high refractive index and a high modulation for compatibility with the ROM, there have come into wide practical use organic dye materials that are melted or decomposed by light having a wavelength within the wavelength region of the laser beam used for recording and reproducing data, thereby enabling the refractive index of the organic dye material to change from a high value to a low value.
In the next-generation type optical recording disk employing a bluish-violet semiconductor laser beam having a wavelength of 390 to 420 nm for recording and reproducing data, it is difficult to increase the reflective coefficient of a data rewritable type optical recording disk using a phase change material as the material for the recording layer to substantially the same level as that of the ROM. The specifications established for the next-generation type optical recording disk therefore designate a low reflective coefficient. Since, unlike in the CD-R or the DVD-R, a high reflective coefficient is not required, it is possible to employ as the material for the recording layer of a write-once type optical recording disk an organic dye material that changes from a low refractive index to a high refractive index when melted or decomposed by irradiation with a laser beam. Japanese Patent Application Laid Open No. 2001-273672 suggests this possibility.
However, no organic dye material has yet been found whose refractive index changes from a low value to a high value when melted or decomposed by irradiation with a laser beam for recording and reproducing data that has a wavelength of 390 to 420 nm.
Further, unlike in the CD-R or the DVD-R, it is difficult to make the long wavelength side of the absorption spectrum of an organic dye material coincide with the wavelength region of 390 to 420 nm.
Furthermore, although ultraviolet ray absorbing agents whose absorption spectrum on the long wavelength side is within the wavelength range of 390 to 420 nm are available, they have a short conjugated system, in other words, are composed of small molecules. Since their solubility in an organic solvent is therefore generally low, these ultraviolet ray absorbing agents are not compatible with the spin coating process and tend to crystallize when used to form a thin film.